Airflow Calculation for Ventilation Systems
The screen copy from our Build Equinox site was taken on a Saturday morning. At that time, CERV homes spread across the country had a collective carbon dioxide concentration of 612ppm. Of course, some homes are higher and some are lower, but when you visit our site, you will most likely find the CERV Community to average 600 to 700ppm. What does this reading mean in terms of the amount of ventilation airflow? How does one calculate the airflow required to meet a desired level of air quality in your home?
Ventilation doesn’t need to be a mysterious chart or a set of intractable, IRS-like, tax code calculations. Let’s break it down into terms that you can use to determine ventilation airflow.
Fresh airflow and the change of carbon dioxide (CO2) concentration in a building space are related to the generation rate of CO2. For all-electric homes without internal combustion sources, humans generate the carbon dioxide. Assuming a home’s adult occupants to be sedentary “plus” (somewhere between watching television and washing the dishes), means that they are exhaling approximately 0.04kg per hour of CO2. With a bit of analyses and unit conversions, we find the following result:
Airflow(liters/second) X CO2(ppm) Concentration Change = #People X 6000 [ppm-liters per second per person]
Current building ventilation standards, which are based on odor rather than pollution, result in an average CO2 concentration change of 600ppm to 700ppm above atmospheric CO2 concentration. For 1 person in a room with an increase of 600ppm-CO2 above outdoor conditions, the above relation shows us that an airflow of 10liters per second will keep indoor air at an average of 1000ppm (400ppm + 600ppm). To those who are used to working with “cfm” (cubic feet per minute) airflow units, multiply by 2. That is, 10l/s is 20cfm with reasonable engineering accuracy, and certainly, with more accuracy that anyone can measure airflow in the field.
Let’s look two examples.
A home has two adults and a half grown child (note: respiration, metabolism, and CO2 exhalation rates are related strongly to body mass….you can use this on your pets, including chickens….yes, we have chicken respiration data). They are home 14 hours per day. What is the average daily ventilation rate required in order to keep indoor CO2 concentration below 700ppm (300ppm above an atmospheric level of 400ppm)?
Average Daily Airflow = 29liters/sec = 60cfm
Note that our chosen target of 700ppm of average CO2 concentration requires twice the airflow needed for an indoor air CO2 concentration of 1000ppm. You are much smarter in an atmosphere of 700ppm of CO2 than 1000ppm.
Note that the fraction of the day (ratio of hours occupied to 24 hours in a day) is used in the calculation. A home with continuous occupation requires more ventilation than a home with less than continuous occupation. But, during the home’s occupancy, each home requires the same fresh air flow.
If the home in Example 1 has a constant ventilation airflow of 60cfm evenly divided evenly among 6 rooms, how high can the living room CO2 concentration reach if everyone is in the living room?
We use the same relation to determine the change of CO2 concentration when we know the airflow into a space.
Living Room CO2 Change = 3000ppm!
Adding 400ppm for atmospheric CO2 = Total CO2 = 3400ppm
Clearly, with constant, low flow ventilation systems, occupied rooms have the potential to become very polluted while air flowing to unoccupied rooms waste fresh air. In future columns, we will address how a smart ventilation system keeps air quality excellent throughout a home.